Direct acylation of phenolic C—H bond usually attracts much attention since phenols are one of the most important aromatic compounds in nature and industry. Two major categories of pathways can be followed in this transformation, namely, the most well-known powerful Lewis acid-catalyzed Friedel-Crafts acylation pathway, and the transition-metal-catalyzed C—H activation pathway. However, Friedel-Crafts acylation reaction is usually difficult to control the regio- and/or mono-selectivity if no directing group is present in the benzene ring, and it often suffers from the harsh reaction condition, usage of air/water sensitive Lewis acid and incompatibility with many functional groups. Meanwhile, although transition-metal-catalyzed C—H activation can direct converting carbon-hydrogen bond into carbon-oxygen, carbon-nitrogen, carbon-halide, carbon-sulfur, or carbon-carbon bonds, till now, there is no report about C—H activation in acylation reaction of unprotected phenols. To our knowledge, only one work has been published reporting the acylation of 1-naphthol with benzaldehyde catalyzed by Pd(OAc)2 in the presence of triphenylphosphine. In that paper, Miura and coworkers didn't carry out an in-depth investigation of the reaction scope, and the reaction conditions were not optimized, while they just used this reaction as a control when they synthesized benzofuran-2(3H)-ones.
Xanthones are important structural units in organic chemistry and widely presented in natural products, and their derivatives were reported to show diverse physicochemical and pharmacological properties such as antioxidants, antiinflammatory, antineoplastic, and vasodilator. Albeit many methods are available for their syntheses, most of them either require advanced starting materials, involve multistep transformations, or exotic reaction conditions, more frequently, via the Friedel-Crafts reactions. There were only a few one-step synthesis of xanthones existed in literature. For example, Larock et al. reported the one-pot synthesis of xanthones by the tandem coupling-cyclization of arynes and salicylates. The same group also reported a C—H activation approach where an arylated imidoyl palladium intermediate promoted the intramolecular arylation resulted in xanthone skeletons. Another elegant work from Li group showed that 2-aryloxybenzaldehyde can undergo an intramolecular cross-dehydrogenative coupling reaction to form xanthones smoothly.